This invention relates to jet aircraft purging fluids. More particularly, this invention concerns itself with an enclosed, continuous process for separating low flash point hydrocarbons from a purging fluid also containing high flash point hydrocarbons in order to restore both fluids to their original quality for reuse.
Currently, maintenance procedures require the defueling of jet aircraft before the aircraft can be placed in a hanger for repair and maintenance. In the defueling procedure, as much jet fuel as possible, such as JP-4, is removed from an aircraft's fuel system and then a purging fluid or liquid is added to the fuel tank. The purging fluid is used to scavenge or remove the last amounts of jet fuel contained in the fuel tanks by defueling the purging fluid. The JP-4 contaminated purging fluid is then placed in a storage tank.
At the present time, the U.S. Air Force utilizes two purging fluids for defueling jet aircraft. The first purging fluid is JP-5 which conforms to Military Specification Standard T-5624 (the same standard governing grade JP-4 fuel) and is also a jet aircraft fuel. The other purging fluid is one that conforms to Military Specification Standard F-38299. This is a narrow cut of less volatile hydrocarbons and is frequently referred to as Phillips 200, Soltrol 200 or 200 fluid.
Insolfar as the purging of the jet fuel itself is concerned, the success of the procedure will be better using a higher flash point purging fluid. The required flash point for Military Specification Standard F-38299 purging fluid is 200.degree. F. minimum. Whereas the flash point requirement for JP-5 is 140.degree. F. minimum. JP-4 jet fuel has a flash point of 0.degree. F. or lower.
The principle reason for purging fuel from a jet aircraft is to render the aircraft less susceptible to explosion when brought into a hanger for repair or maintenance. After defueling jet fuel, such as JP-4, from an aircraft, a residual of JP-4 liquid and its volatile vapors persist in various recesses and inter- connections of the total fuel system. This is particularly true of the smaller fighter type aircraft rather than the larger bombers and cargo planes because in the fighter type aircraft the fuel is not only located in the wing structure but also throughout the fuselage. By refilling a fuel system with purging fluid, the residual JP-4 is mixed and dissolved in the purging fluid. The admixed purging fluid is then drained from the aircraft fuel system leaving behind a residual of purging fluid slightly contaminated with JP-4 to replace the undiluted explosive JP-4. The explosiveness of the aircraft after purging, therefore, is substantially lowered with the degree of explosiveness being a function of the flash point of the admixed purging fluid used. Should a purging fluid be used a second time, the quality of the purging process on the aircraft will not result in as low an explosibility limit as when the purging fluid was first used. This is due to the fact that as the purging fluid is used and re-used, it becomes more contaminated with JP-4 which tends to lower the flash point tremendously.
Another procedure used to minimize the explosive potential of admixed purging fluid is to aerate the purging fluid in the storage tanks between its reuse in the defueling procedure. Aeration removes some of the more volatile constituent hydrocarbons of the JP-4 jet fuel and thus helps to retain the quality of the purging fluid and extend its life for additional purging cycles. This is accomplished, however, at the expense of contaminating the surrounding atmosphere.
Without aeration, a purging fluid will drop in flash point by as much as 20% with one percent JP-4 added. Even with aerating the purge fluid between uses of it, there comes a point at which aeration does not raise or improve the purge fluid's flash point due to an accumulation of intermediate volatile hydrocarbon constituents in the JP-4 jet fuel.
It becomes obvious from the problems referred to above, that the development of a novel and more efficient system for reclaiming purging fluid and the JP-4 contaminant would be of great value. As a consequence, a considerable research effort was undertaken in an attempt to provide a method which minimized the explosive character of reclaimed purging fluid while at the same time overcome the problems of atmospheric contamination.
With the present invention, a closed, continuous, vacuum distillation system has been found which provides for the regeneration of the purging fluid to its original condition, or even to a higher flash point, if desired, and the reclamation of the JP-4 jet fuel rather than allowing it to escape into the atmosphere as a contaminant.